It comes from Wright v Michael Wright Supplies Ltd & Anor  EWCA Civ 234 (27 March 2013), a dispute between businessmen. Enjoy!
“You may be able to drag the horse (a mule offers a better metaphor) to water, but you cannot force the wretched animal to drink if it stubbornly resists. I suppose you can make it run around the litigation course so vigorously that in a muck sweat it will find the mediation trough more friendly and desirable. But none of that provides the real answer.”